In telephone systems of the types in use today, relay circuits are used to detect line condition to signal that condition to the exchange. The relay provides isolation between the input and output paths, as is well-known.
Optical couplers or isolators using light emitting diodes have become popular in the recent past for performing line current detection for the resultant signaling. For example, U.S. Pat. No. 3,941,939 issued Mar. 2, 1976 to P. Holmes et al. shows an optical coupler or isolator to sense line condition for ring trip control. Prior patents such as U.S. Pat. No. 3,748,401 issued July 24, 1973 to R. Pesz et al. and U.S. Pat. No. 3,865,992 issued Feb. 11, 1975 to L. Bouty et al. show this principle also for the line condition sensing for ring trip.
The use of liquid crystal devices to provide condition sensing by the use of optical means is shown by U.S. Pat. No. 3,718,381 issued Feb. 27, 1973 to G. Assouline et al., U.S. Pat. No. 3,627,408 issued Dec. 14, 1971 to Fergason, U.S. Pat. No. 3,837,730 issued Sept. 24, 1974 to W. Hatfield et al. and U.S. Pat. No. 3,920,996 issued Nov. 18, 1975 to Moore.